Hi,
there could be several reasons. Landing gear still down, cruising at low altitude, wrong speed, not all tanks filled,...
The plane will carry 16.000 kgs of fuel, that is enough for about 5+ hours of flight. Your fuelflow should be about 2200-2500 kgs/hour (both engines) with a speed of ca. 420kts. If you don´t get these values, you are doing something wrong :-)
Cheers, Jan